The Ghost in act 1, scenes 4 and 5, is thought to be an evil omen because its true nature and its intentions toward Hamlet are unclear. This idea is developed both before, during, and after Hamlet’s interaction with his ghostly father. Upon seeing the Ghost for the first time, Hamlet doubts its form and intention, stating,

Be thou a spirit of health or goblin damn’d
Bring with thee airs from heaven or blasts from hell,
Be thy intents wicked or charitable,
Thou comest in such a questionable shape
That I will speak to thee.

Although he is aware that the Ghost may in fact seek to bring him harm, Hamlet proceeds to meet with it alone against the advice of both Marcellus and Horatio. When Hamlet states that he does not fear the ghost because he does not fear death and that he believes the Ghost cannot harm his soul (although it eventually does just that), Horatio asks Hamlet if the Ghost might “might deprive your sovereignty of reason / and draw you into madness.” Hamlet responds that his “fate cries out,” and he sets that fate by following the Ghost, who ultimately puts him on the path that Horatio warned him about. Soon after meeting the Ghost and before even hearing that his uncle was the murderer, Hamlet quickly abandons reason in his hope for revenge:

Haste me to know’t, that I, with wings as swift
As meditation or the thoughts of love,
May sweep to my revenge.